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Equatorial Kelvin and Rossby Waves Evinced in the Pacific Ocean

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Equatorial Kelvin and Rossby Waves Evinced in the Pacific Ocean 3 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, SUPPLEMENT, PAGES 3249-3262, FEBRUARY 28,1991 Equatorial Kelvin and Rossby Waves Evidenced in the Pacific Ocean Through Geosat Sea Level md Surface Current Anomalies Crow SURTROPAC,Instirut Frangoir de Recherche Scientfiue pour le D¿vclopment en CwpLration (ORSTOM).Nod, New Cale&& Almaet. Equat0ri.l Kelvin and Rosiby wave am comprehensively deanonmtcd over most of the quatoriaa Pacific bin, through their ~ignamnri sa level and mal surfaœ gwrqhic cWrent anomalies. This was made possible with altimeter data peruining to the First year of the Gemat (Geodetic Satellite) l7day exact repuu &i (Novanber 8.1986. to Navaaiber 8.1987). To thin end, alcmg-trpck m~edGemat rea level lnomaliu (SLAm). &ve to the time period of interest, wem fitmooched using nanlinur and hear fílim. The opiginal17-d.y time step aras th duadby canbming dl ascending ~d descending tracks within 10" Ionghdiialbmds. Fay,SIAS wem gridded onto a mgular grid. and 1ow-pIps filters were applied in lanitlade and time in onder to mroolh ollt miliraing high-frequency noire. Anomalia of mal surface geostqhic aumat we= ulculucd uahg the fim amd zecmd derivatives of ?he SLA meridimal gradient, off and on the equator, rcrpcdively. .Ch surface cum1 mdea validated in the wcrtem quataid Pa&k ' level and am whb in situ data grchen8 bring rewn hydrognfic cruises at 165"E. and through expendable , b.thy&emogrsph prpd mooring mensuranam. Following their chrckldogicd oppea~cedong the 165% maidiaa, rhe mapr low-frequencySUS mB und aurfaa cunant anomalies am described and explained in termo d &e equa(0ri.l wave theory. An quataid dovvlrwelling Kelvin wave, known to be the main dc signal d the 1986-1981 El Nao. ir gaiemted in December 1986. concomitllnt with a strong westerly wind 1 anomaly ocarning wert of the dateline. The asmdued propagating equated SLAs compdto an elevatim d IS an. Wcpmht utimues of this Kelvin wave phase rpeed are obtained through time-lag codation mclarix dyrh0.82 f O.% m a") and the lust quam fit of &e SUmeridimd mmhlum to theorclical Kelvin wave [email protected] f 1.02 m s-l). Boch estimates indicae lhat the Kelvin wave has the dumueristic of a fiest brpoctinic made. An quatorinl upwelling Kelvin wave is then hblein June 1987. It is charrclerized by a loCm eea LevJ w,pmpgating only fmi Ihe western to the cenaal quatorial Pacific. A first "d.. mode (-1) quuaial upwelling Roraby wave ming&e pntire Pdichsin from March 1987 (mmput) DO SeQIEmber 1967 (wutcm pan) rhowr up in SUr urd mal dacecurrent anmllies. Such a hrby wave catupads to pmpgatinp sa kvd drops which are cxuune (-12 an) at Iboclt 4" and 4% latirudw. The OanrequenCM o(p mdsurfaœ geortrophic QlhRllt are very important lince, in the cllsc of the upwelling, it dnmatidy degeam the thpte major surface cu" (the Ndmd South Equaorial CaMte~cumntr,and !h?h Equatorial Current] by m amplitude similar to thkir mean annuat velocity values. The lust aquas fit of the Rorrby wave SEA meridid amdues to its thwwtid ml fo~mcqenlly rugguts the dominance of the first banodinic de(~2.59 f 0.65 m 1.3. This danhaneis corrobowted by .II ulitnue of Ille Rsshy wave phare (1.02 f 0.37 m s-1). whish mghly ma phue cpeed (c/2mtl) a€ &e m1 quaaial Rocsby wave. It ir suggested &at the qpIopial upwelling Rossby wave b mdydue to a reflectiora of an equutorial upwelling Kelvin wave generube8 in January 1987 nur the dateline. Whahcp op not the overall prqwguing features am pan of rhe 1986-1987 El Neo a belmg to the "nomul" malcycle cannot be decided in the ahced longer altirmer p6. level time mies. 3249 ORSTOM Fonds Dosumenkaire ,a. -,** Nog SLeJg%,M.A n i p 43 3250 DELCROXET AL.: EV~ENCBOF EQUATORIAL KELVIN mn ROSBYWAVES of obtaining basin-wide sea level observations over a long the moved to a 72" orbit inclination for the 17-day EM,which period. Hence it plays a major role in quasi-real-time became fully operational on November 8, 1986. The ERM monitoring of the oceans (cf. Climate Diagnostics Bulletin, collinear ground tracks have an equatorial separation of 164 km published by the Climate Analysis Center, National (82 km with the combination of ascending arad deswdiing Meteorological Center, National Weather Service, NOAA, tracks). and the original 0.1-s sampling altimetric measurements Washington, D.C.) and the past and anticipated results meet averaged every 1-s result in a 6.8-kirn dong-back resolution. most of the goals expressed in cliiate-related programs such as The data used in the present study correspond to the fist 22 the Tropical Ocean and Global Atmosphere (TOGA) and World repeated cycles (November 8, 1986. to November 18. 1987). Ocean Circulation Experiment (WOCE) programs [cf. Srewart They were processed and kmdly provided to us by Chet &Lefebvre. 19871. Kobliiky of the NASA Goddard Space Flight Center (NASA Chew and Miller 119881 demonstrate the usefulness of GFSC). The following briefly presents the corresponding Geosat in describing the sea level variability during the processing. The along-track data were ae first interpolated using 18-month classified geodetic mission and approximately a tenth-order Hermite inteplation algorithm. In a way similar 7-montli Exact Repeat Mission (EM). Tai et al. [ 19891 verify to that of Ckmy et al. [1987] the albimetric sea level data were a level versus dynamic height field corrected from tides [Schwiderski. 19801, sea state bias, wet athythermograph (XBT)and island troposheric refraction. and sea level pressure (the last two sea level data. Miller ee 01. [1988] detect equatorial Kelvin corrections based on the Navy Fleet Numerical Oceanographic wave propagation during the 1986-1987 El Niño / Southern Center model output). The satellite orbit was computed by the Oscillation (ENSO), and Miller and Chaney I19901 present Navy Astronautics Group, using Doppler Racking from three results showing large-scale sea level fluctuations and changes in stations. Following the technique recammended by @heprey et upper layer volume in the tropical Pacific during the 4-year al. [%983]. radial orbit error was removed from the mean period from April 1985 to Fbruary 1989. To date, there has collinear track detrended over 2588-km segments through the derive geostrophic zonal surface least squares fit quadratic function. In order to eriminate the Feòsat measurements. Picaut et al. mean geoid, the Final sea level data set corresponds to collinear [1990], through a comparison with in situ near-surface zonal deviations from the means calcuIated over at least 20 out of the current, show that meaningful geostrophic surface current 22 original cycles. Data gaps, evident on Figure 1, are either estimates can be deduced from Gasat sea level, right at the due to less than 20 complete cycles or to data rejections equator. Our present investigation complements and tries to go resulting from standard deviation criteria over the original 0.1-s further than these Geosat tropical ocean-related studies. data [Cheney et al.. 19871. Owing to the elimination of the The purpose of our paper is, from the Geosat ERM data for mean, the mean sea surface topography is removed, and one the 1986-1987, ENSO period, to investigate cross-Pacific must keep in mind that all the results of the present paper axe propagations of equatorial kelvin waves and, for the first time, relative to the average for the November 8, 1986, to November of an equatorial Rossby wave. These propagations will be 18, 1987, period. Therefore we will only examine sea level documented and analyzed both in terms of Geosat sea level anomaly (SLA) or current anomaly in the following sections. anomalies (GSLA) and suqface geo&ophic zonal current anomalies (GZCA). We will investigate whether these 2.2. Data Processing equatorial waves have the charìxteristics of a first baroclinic Sea level gridding. Although the processed GSLA we mode via several independent phase s estimates. Namely, obtained from NASA GFSC were already corrected for all we will use time:lag correlation analys determine the mnal environmental disturbances, additional filtering in space and propagation. and will adjust the equatorial meridional trapping .time is required to remove remaining instrumental and/or scale of these waves to the theoretical shapes hough a 1 %ceanic noises. Figure 2 exemplifies a 3OoS-30'N along-track squares fit. Tentative explanations for the origin of GSLA, and clearly evidences tlie need to suppress nonphysical equatorial Kelvin and kossby waves are also proposed. We will 'spikes, the largest occurring in the vicinity of the Intertropical show that wind stress anomaly forcing is consistent with Kelvin Convergence Zone (ITCZ). Thus the fist step of ow own wave triggering, and that a fist meridional mode Rossby wave GSLA processing is to apply a nonliiear median filter on the observed all the way to the western Pacific may be the along-track data, following Picout et al.'s [1990] technique. reflection of a Kelvin wave at the eastern Pacific coast. These authors tested different filter lengths by comparing The papis orgànized as fo1Iows. Ira section 2 the processing equatorial mnd surface geostrophic currents derived from of altimeter data is discussed, and an evaluation of Geosat sea GSEA and directly memured mats fpom equatorial levels in the western Pacific is presented. In section 3 the moorings. Such comparisons constitute the most strhgent Pest, propagations of equatorial Kelvin and Rossby waves are since, on the equator, the geostrophic currents involving the analyzed according to their sequential appearance in the western meridional GSLA gradient me very sensitive to small-scde equatorial Pacific. The related GZCA are examnined further in GSLA noises.
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